What does the console window say? It could be a problem in the texture path, or setting mipmap levels, or loading the texture image or what else... just examine the console output.
I had a simlular problem with several bmp. After update to 0.5 many (If not all) of my bmp's displayed white in partical systems and an ugly light pinkish orangish color(definiatly not pretty).
I converted the bmp to jpg and everything worked smoothly.
Unless I am a tard I would say this looks like the first 0.5 bug. Or the first F$@#$# up by me with 0.5
This is the same weird error that happened in 0.4.2 bmp's that loaded fine now don't, and for the particles, it says compressed bmp's are not supported by the engine currently, idk if this is the same error you have w/ v 0.5, but this is what happens in 0.4.2. Also, the orange color is most likely the light affecting the billboard.
The Robomaniac
Project Head / Lead Programmer Centaur Force